Automatic frequency control system



April 22, 1952 c MUNSTER AUTOMATIC FREQUENCY CONTROL SYSTEM 3 Sheets-Sheet 2 Filed Jan. 21, 1948 MSESEQQQQ INVENTOR fluff) C. 070/7572)? Ap 22, 1952 A. c. MUNSTER AUTOMATIC FREQUENCY CONTROL SYSTEM I s Sheets-Sheet :5

Filed Jan. 21, 1948 'IIIIIIIIIIIIIIII F/Cf45.

& A;

Patented Apr. 22, 1952 AUTOMATIC FREQUENCY CONTROL SYSTEM Allen 0. Munster, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application January 21, 1948, Serial No. 3,478

. 1 The invention herein described and claimed relates to improved automatic frequency control systems. In particular it relates to such systems for use in radar receivers of the superheterodyne type in which the local oscillator comprises a thermally tunable, velocity modula-, tion type vacuum tube.

In receivers employed in radar systems and which customarily make use of automatic'frequency control to compensate for variations in the frequency of the transmitted'signal, reflections of which are subsequently received from target objects, it is the usual practice for the automatic frequency control circuits to be operated in response to the transmitted signal rather than in response to the received reflections thereof from target objects. This provides much more effective control of the receiver circuitsbecause of the greater strength of the transmitted signals as compared with the received reflections 2 Claims. (01. 250-36) -tain difficulties are encountered owing to the thereof. For this purpose there is customarily employed in the receiver a discriminator of conventional form, supplied with a signal of suitable intermediate frequency which may be obtained by heterodyne frequency conversion, in the usual manner, of a suitably attenuated signal from the transmitter. The output from this discriminator, which is a function of the'departure of the transmitter frequency from a predetermined nominal value, is applied to vary the frequency of the local oscillator in the receiver in such manner that the intermediate frequency, to which the received reflected signals are convertedby means of a converter cooperating with the local oscillator, is maintained essentially constant.

In such a system it has been customary in the past for the local oscillator to comprise a velocity modulation type vacuum tube having a repeller electrode to which the output from the discriminator is applied, whereby relatively smallvariations in the frequency of the oscillator are produced without actual change in tuning of the output cavity of the velocity modulation tube. More recently, however, it has been proposed to employ, as a local oscillator, a velocity modulation type tube in which the actual tuning of the fact that the frequency of the output from the tubeis susceptible of variation throughout a relatively wide range, and further owing to the fact that, in a radar system employing, as is customary; a cavity magnetron oscillator as a primary source of super-high frequency energy, a substantial amount of time is required, after the application of power to the magnetron, before it becomes operative at substantial efliciency. During such periods of "warming-up there will be nosignalavailable for supply to the input of the automatic frequency control discriminator and hence there will be no control over the frequency of the local oscillator, which, in general, will reach full efiiciency of operation before'the cavity magnetron which comprises the transmitter. During this period the local oscillator may drift considerably in frequency so that, when the trartsmitter oscillator finally achieves its normal operating level, the value of the intermediate frequency signal applied to the discriminator may not be such as to cause the discriminator to produce a suitable output tending to control the local oscillator frequency to the required value to compensate for the then existing frequency of the transmitter signal. Furthermore, the range of possible variation in the frequency of the output from such a tube is so large that the desired intermediate frequency may be produced when the local oscillator frequency is equal either to the sumor to the difference of the transmitter frequency and the desired intermediate frequency. This leads to difficulty because of the factthatfor only one 'of these values of local oscillator frequency will the output of a conventional discriminator circuit, employed for automatic frequency'control purposes, be in the propersense to maintain the local oscillator frequency at the proper value. For the other value of local oscillator frequency, the output of the discriminator will be such asto produce instabilityof operation-that is, the output of the discriminator will vary in such manner, inresponse to variations in the input signal thereto, as to control the local oscillator frequency away from the desired value rather than toward it. Thus, if, during the period of warm-up, the local oscillator frequency varies throughout a range which includes the undesired value aswell as :thedesired value, and if the local oscillator is operating-in the vicinityof the undesired 'value at the time when the discriminator begins to receive input. signal from the converter, there may be considerable delay before the automatic frequency control circuits commence to operate in the desired manner. In fact it is even possible that such operation may never be achieved and that the local oscillator frequency will continue indefinitely to be controlled away from the desired value. rather than toward it. Moreover, even after the transmitter has attained its normal operating output level, sudden changes in the frequency of its output may occur which will alter the intermediate frequency to such an extent as to render the automatic frequency control circuits ineffectual. In this event, unless some means are provided for controlling the local oscillator so as to restore theintermediate frequency to a value such as to permit the automatic frequency control circuits to regain con,- trol, this undesirable situation may continue indefinitely.

It is the principal object of the invention to provide means for eliminating these difficulties and for insuring that the usual automatic frequency control circuits in a system of the sort just described, employing a thermally tunable local oscillator, will become operative and take over their intended function assoon as possible after the transmitter oscillator has reachedits normal level of operation.

Another object of the invention is to provide, in an automatic frequency control system employing a discriminator of conventional form operative in response to a signal whoseIfrequency is to be controlled, and in which the discriminator becomes ineffectual whenever the signal departs in frequency from a predetermined value by more than a certain amount, means for automatically restoring the frequency of the signal to a value such as to render the discriminator again effectual.

A further object of the invention is to provide, in an automatic frequency control system comprising a heterodyne frequency converter adapted to be supplied with. signal from a local oscillator and with an input signal, and comprising a discriminator responsiveto the outputfof said con-. verter for controlling the frequency of said local oscillator whenever the frequency of output frorn said converter lies within, a predetermined range,

means operative whenever the frequency croutput from said converter lies outside. saidprede termined range for controlling said local oscillator in a manner tobring said output frequency again within said range.

In accordance with the invention these objectives are achieved through the provision of means for precisely controlling and limiting the range of frequencies through which the outputfrom the thermally tunable local oscillator varies during the period of warm-up of the transmitter-oscillator and at all times when the normal automatic frequency control circuits are. ineffectualbecause of the fact that the intermediate frequency signal supplied to them is not of a suitable .value. The range of variation, to which thefrequency of the output from the local oscillator is thus constrained, is made sufliciently wide to take account of any normal variations in the intermediate frequency owing to variations in the frequency of the output from the transmitter oscillator, butis limited so as to avoid the possibility of producing the intermediate frequency by such a combination of the transmitter frequency and thelocal oscillator frequency that the output of-the automatic frequency control discriminatorwill vary in such of the local oscillator is caused to sweep back and control circuits to commence functioning. When this occurs, the output of the discriminator is applied to control the frequency of the local oscillator and the means provided for sweeping the frequency of the local oscillator during the warmup? period are rendered inoperative.

Accordin ly-another object of the invention is to provide novel and improved means for precisely controlling the frequency of an oscillator and for during this period of "warm-up the frequency constraining it to vary within a predetermined definite range.

The method by which these results are achieved and the means for accomplishing them wll1j be more fully understood from a consideration of' the following detailed description with reference to the accompanying drawings in which:

Fig. l is a block diagram of a conventional automatic frequency control circuit to which the invention is applicable,

Fig. 2 is a diagram which will be referredto in explaining the principle and mode of operation of the invention,

Fig. 3 is ablock diagram of a typical embodiment of the invention,

Fig. 4 is a schematic diagram of a system in accordance with the invention, and

Figs. 4A and 4B are diagramsillustrfitin certain structural details of the systemaccording to Fig. 4.

Before proceeding with a detailed description of a typical embodiment of the invention, it will be helpful first to consider in furtherdetail the nature of the difficulties, to be eliminated. This can most readily be done by referenceto; Figs, 1 and 2 of the drawings. Fig.1 shows an,elem entary automatic frequency controlsystemin which a converterl is supplied withinput 1signal,;for example, from the transmitteroscillator of a conventional. radar system. Converten l jis also supplied through connection 2 with local j-oscil-v lator signal from local oscillator 3,,the. .output frequency from which. isassuined to be variable throughout such a range that the .desiredinteI--. mediate. frequency .of the output from, converter, I, which is equal to.the differenceinjrequency between the output-from local oscillator. 3 .and the input to converter. l fromrthetransmitter oscillator, may be obtained for either of .two yalues of local oscillator frequency within said-range namely, when the local oscillator. frequency is either greater or less infrequency; than the input to converter l by an amount equal tothe-inter mediate frequency. The intermediate-frequency output from converter 1 is suppliedthroughconnectionA to a conventional discriminatorcircuit 5. The control voltage developedibyadiscrimina: tor 5 is supplied-through ,connectionfi .toQcontrol the frequency of local. oscillator? Local=oscil1 lator 3, for example, may be a thermally tunable velocity-modulation type vacuum tubelof the sort hereinbefore mentioned.

In Fig. 2 is'shown a plot of the-output of -dis criminatorb in the arrangementaccording to, Fig. 1 as a function of the output frequency of local oscillator 3. The solid line: in ,theupper t o o this; fig e ar p entsrt e v i t ons in the discriminator output with local oscillator frequency for a particular value of the frequency of the input from the transmitter oscillator to converter 1 of the arrangement according to Fig. 1. It will be noted that this curve has two crossover points at frequenciesfa and it displaced respectively below and above the transmitter oscillator frequency fc by the value of the desired intermediate frequency fi-f. It will further be noted that the discriminator output in the vicinity of fa is positive below and negative above fa, while, in the vicinity of fb, the discriminator output is negative below and positive above It: If the discriminator output in the vicinity of fa is such as to control the local oscillator toward fa, as indicated by arrows I, then the discriminator output in the vicinity of fn will be such as to control the local oscillator frequency away from fb, as indicated by arrows 8, and thus to produce instability of operation. As indicated above, this curve represents the discriminator outputs only for a particular value of frequency of the input to converter I from the transmitter oscillator. In practice, of course, this frequency will vary and hence the discriminator output curve will, in effect, move up and down along the frequency axis throughout a predetermined range depending upon the range of variation of the output frequency of the transmitter oscillator. For example, the range of variation of the transmitter oscillator frequency may lie between frequencies f0 and f0". If the transmitter oscillator frequency assumes its extreme lower value 12;, the discriminator output curve will appear as represented b the broken line and will have crossover points at frequencies fa. and it respectively. On the other hand, if the transmitter oscillator frequency assumes its extreme upper value fc" then the discriminator output curve will be as represented by the dotted line and will have crossover points at frequencies fa" and ft" respectively. It will be obvious therefore that, in order for the automatic frequency control circuit to become operative, the frequency of the local oscillator should have some value within the range f -f the upper and lower limits of which are determined respectively by the two extreme positions of the peaks-of the left-hand portion of the discriminator output curve. On the other hand, for the reasons hereinbefore indicated, the local oscillator frequency must not-be permitted to enter the range f 'f According to the invention it is proposed therefore to provide means for sweeping the frequency of the local oscillator back and forth in the range f 'f during the period before the transmitter oscillator reaches its normal level'of efficiency, whereby, at some time after it reaches this level, there will be produced an intermediate frequency signal such that the output from the discriminator in response thereto will cause the local oscillator to seek the proper output frequency to yield the desired intermediate frequency. The invention also contemplates means responsive to the output from the discriminator for inactivating the sweeping means when the automatic frequency control circuits have become effective. v

The means for accomplishing these results in accordance with the invention will now be generally described with reference to Fig. 3 of the drawings. The resemblance between portions of the arrangement of Fig. 3 and that of Fig. 1 will be noted. The system according to Fig. 3 comprises a converter 9 supplied with input from the transmitter oscillator of the radar system. and

also supplied through connection II] with local oscillator signal from local oscillator H. The intermediate frequency produced by converter 9 is supplied through connection I2 to a discriminator [3. The output from local oscillator H is also supplied through connections I4 and I5, respectively, to cavity resonators l6 and H, which are tuned, respectively, to the upper and lower limits of the range throughout which the local oscillator frequency must vary in accordance with the requirements as hereinbefore set forth in the discussion of Fig. 2. That is, cavity resonator l6 may be tuned to the upper limiting frequency f while cavity resonator I! may be tuned to the lower limiting frequency ,f Thus, when the output frequency of local oscillator ll approaches the frequency to which resonator I6 is tuned, that resonator will be excited; while, on the other hand, when the output frequency of the local oscillator approaches the frequency to which resonator I1 is tuned, that resonator will be excited. Associated with the resonators l6 and [1 are suitable detectors Ilia and Ila for de-.

veloping unidirectional potentials whose magnitudes are functions of the extent of excitation of the particular resonators with which they are respectively associated. These detectors, for example, may comprise crystal diodes appropriately coupled to the cavities and differently connected, so as to develop potentials of opposite polarities in response to excitation of their respective cavities. The output leads from these detectors are connected to a signal adding device I8,'from the output of which there will be derived a potential whose polarity will be positive or negative depending upon which of the resonators is excited. This potential is supplied through connection 19 to control the operation of a sweep multivibrator 20. The output from sweep multivibrator 2|] is supplied through connection 2| to a switch 22, which is also supplied through connection 23 with the output from discriminator I3. Switch 22 is further controlled through connection 24 from discriminator l3 so that, in the absence of output from discriminator l3, switch 22 interconnects connection 2| with connection 25 so that the output from sweep multivibrator 2B is supplied to local oscillator H to control the frequency of the latter. Sweep multivibrator 23 may be of conventional form, having two different conditions of output in its operating cycle. The output corresponding to one of these conditions may be used to cause the frequency of local oscillator II to sweep in one direction; while the output corresponding to the other condition may cause the local oscillator frequency to sweep in the other direction. Moreover, the output from voltage adder I8, depending upon its polarity, may be used to determine which of these output conditions exists at any given time. Thus, assuming the output from multivibrator 20 to be such as to cause the local oscillator II to sweep from a lower to a higher frequency, when the frequency of the 10- cal oscillator reaches the upper limit of the desired range, high frequency resonator 16 will be excited in response to the output of local oscillator II at the upper frequency limit, this will produce a potential of predetermined polarity in the output of signal adder l8 and this potential, supplied through connection H! to multivibrator 20, will cause it to assume its other output condition and to commence sweeping the local oscillator-downward in frequency. When the output frequency of the local oscillator reaches the lower limit of the range, low frequency resonator I! will be excited, a potential of opposite polarit will appear in the output of signal adder l8, and this will again cause the multivibrator to change from one to the other from sweep multivibrator 20 to local oscillator ll. When this occurs, the frequency of the output from local oscillator II will no longer be swept back and forth in response to the output from sweep multivibrator 20 but will be controlled in the desired manner in response to the control signal developed by discriminator l3. Thus there is provided a system in which, during the warm-up period of the transmitter oscillator, the frequency of the receiver local oscillator is swept back and forth throughout a range which encompasses all of the potentially desirable values thereof required to yield the de sired intermediate frequency, and in which, when the transmitter becomes operative and a suitable intermediate frequency signal becomes available to operate the discriminator, the sweeping action is discontinued and the output of the discriminator is threafter used to control the local oscillator.

Having described in general the principles underlying the invention and the mode of operation of a system in accordance therewith, there will now be described in detail a representative embodiment of a system according to the invention as applied to a typical radar system. In Fig. 4 radar transmitter M is actuatable in response to power supplied to it from power supply 33 upon the closing of switch 39, and is controlled b a pulser 40 to produce time-spaced pulses of super-high frequency energy. These pulses are supplied through a waveguide 42 to a suitable transmitting and receiving antenna #3. T he latter transmitts this energy and also intercepts reflections thereof from target objects,

which are transmitted through waveguide section 42, T-R device :25 and waveguide section 45 to a crystal mixer 51. The latter is also supplied through waveguide section 18 with local oscillations from thermally tunable, Velocity-modulation type local oscillator tube at. Crystal mixer 41 operates to convert the signals thus supplied to it to a suitable intermediate frequency signal which may then be supplied through transmission line 53 to the usual radar receiver (not shown). It is to be noted that, in accordance with prior practice, an anti-T-R device is connected to waveguide section 42 for the purpose of preventing loss of received signals to the radar transmitter 4|.

Signals from radar transmitter M are also supplied from waveguide 42 through a suitable attenuator 53, and waveguide 54, to auxiliary crystal mixer 55. The latter is also supplied through waveguide 56 with signal from local oscillator 49. Crystal mixer 55 functions to convert theradar transmitter signals supplied to it to a suitable intermediate frequency, which is the same as that produced by crystal mixerd'l. Received signals from antenna 43 are, ofcourse;attenuatedto such an extent, by attenuator 53, that, in effect, none of them appear at mixer 55. This intermediate frequency signal is supplied through a conventional intermediate frequency amplifier 51, including a final stage comprising tube 58, to a discriminator circuit 62. The latter comprises diodes 59' and 86 and a plural secondary transformer St, the primary winding of which is included in the plate circuit of I.-F. amplifier tube 58. This circuit is conventional in form, the secondaries 61a and Gib of transformer 6| being tuned respectively to frequencies slightly above and slightly below the desired I.-F. frequency. The operation of this form of circuit is well understood by those skilled in the art to which this invention relates, and accordingly need not here be discussed in detail. The discriminator output is developed across resistors 63 and 64, connecting the cathodes of diodes 59 and 68, and is supplied through an amplifier 65, adapted to produce an even number or" phase inversions, to connection 66 between the cathode of tube 61 and the plate of tube 68. The plate of tube 6'! and the cathode of tube 68 are connected together by means of connection tea, and both are connected through resistor To to one plate of condenser ll, whose othei' plate is grounded. Tubes 61 and 68 and the associated connections, just described, comprise a gate and integrator circuit 69. In response to pulses supplied from pulser 43 through connection 12 and transformers 13 and M to the grids of tubes 61 and E8, the latter are biased to conduct during intervals coextensive with the transmission of pulses by radar transmitter 4 l If, during these intervals, the potential applied to connection SE is positive, current will flow through tube 68 and resistor 79 into condenser I, thereby increasing it charge. 011 the other hand, if, during these intervals, the po tential applied to connection :35 is negative, current will flow from the condenser through resistor "ED and tube 6'! in a manner to reduce the charge on the condenser. The ungrounded plate of condenser H is connected to the grid of a cathode follower tube 15, and a connection '26, including an isolating resistor 16a, is provided between the cathode of tube 15 and connection 66. By reason of this connection, in the absence of discriminator output supplied to connection 66 from amplifier 55, connection 56 will be maintained at a potential such that current will flow in neither tube 6'! nor tube 68, and hence condenser H will tend to maintain its charge. If, however, the potential applied to connection 66 during the intervals corresponding to transmitted pulses includes output, either positive or negative, from discriminator '62, current will flow in one or the other of tubes 61 and 68, thereby tending to modify the charge on condenser H in accordance with the discriminator output. Thus there is developed across condenser (I a potential which is determined by th output from discriminator- 62 during the intervals corresponding to the transmission of pulses by the radar. Between such intervals the potential across condenser Tl tends to remain substantially constant except as modified by any fortuitous leakage of the condenser.

'Ihe potential developed across condenser TI is supplied through connection H to the control grid of a 'p'entode '18, which, together with another pentode l9, forms a switch circuit 80. There is also provided a connection 8| from a load resistor 82, in the plate circuit of I.-F. am-

plifier tube 58, to the-grid of aswitch control tube 83 This connection includes resistor 09, whose function will be explained presently.- Switch control tube 83 is provided with load resistors 84 and 85 connected respectively in-i'ts plate and cathode circuits. There is a connection 85, including resistor 86a, from the plate of tube 83' to the suppressor grid of switch tube I8. 'Also there is a connection8I, including resistor 87a, from the cathode of tube 83 to the suppressor grid of the other switch tube I8. Condenser 88, by-passing resistor 82 in the plate circuit of tube' 58, is chosen so that the timeconstant of the R. C. circuit comprising resistor 82 and condenser 88 will be long compared to the durations of intervals between transmitted radar pulses;- Accordingly, when the radartransmitter is operating and input is being supplied to discriminator 62, a moreor-less constant negative potential will be developed across condenser 88. Any tendency of this potentialto vary at the radar pulse repetition rate may be further reduced by means of a filter circuit comprising resistor 89 and condenser 90, the values of which are likewise selected to provide a time constant long compared to theintervals between transmitted radar'pulses. It is also to be noted that the-value of resistor 89 is to be selected with a view to providing appropriate plate potential for tube 56 and suitable grid bias for tube 83. Thus, when discriminator 62 is functioning, a negative bias; will be applied through connection BI to thegrid'of switch control tube 83 and willter'ld to reduce the current flow through the latter, thereby reducing its cathode potential and increasingits plate potential. By reason of connections 86 and 91 from tube03 to the suppressor grids of tubes I8 and I9, signals applied to the control gridof switch tube 18 will be caused to appear-across resistor I2I, but signals applied to the control grid of tube I9 will not.

Output from thermally tunable local oscillator 49 is also supplied through waveguide sections 95 and 96, having suitable wideband terminations 95a and 96a, to cavity resonators BI and 98,

which may be of conventional cylindricalcon- 4 figuration and which 'aretuned respectively to the lower and upper frequency limits of the range throughout which the frequency of the local oscillator 49 must vary in accordance with the principles hereinbefore set forth. Couplingfrom.

cavity 490. of local oscillator tube 49 to'wave guide sections 95 and 96 through ashorttransmission line section WI and capacitive probe 1 02. Coupling from waveguide sections 95 and 96 to cavity resonators 9I and 98 ,is through couplingplunger IIO .in ,waveguidefs'ections I05 and I06 are disposed crystal diodes 99 and I00, which are adapted to respond respectively to the excitation of resonators 9I and 98, energy from one of said resonators, when it is excited, being transmitted to the corresponding diode through one of waveguide sections I05, I06. Plungers I I may, of

course, be adjustedtoproduce maximum output from each diode. Diodes .99 and I00 aredifferently disposed within their respective waveguide sections I and I06 so that their respecinthe tuning ofcavity resonator 49a through the tive outputs will be of opposite polarities. actual arrangement of the diodes within wave- The guide sections I05 and I06 in typical apparatus is indicated in Figures 4A and 413 respectively.

These figures are self explanatory;

The output terminals of diodes 99 and I00 are connected through a common load resistor III "to ground. There is a connection from the ungrounded terminal of load resistor I II to the grid 'of a multivibrator control tube II2, having load "resistor I I4 through a cathode-coupled tube II8 tothe grid of the other multivibrator tube H6.

The output of multivibrator 1, derived across plate load resistor H9, is supplied through connection I20, including resistor I20a, to the control grid of switch tube I9. The control grid of provide suitable grid bias for tube I9.' The output of switch circuit 80, developed across a common resistor I2I in the plate circuitof switch tubes I8 and I9, is supplied through transmission' line I22 to the thermal control grid of suitable bias is applied to this grid through the cooperation of resistors I23, I24 and I2I. As is well known, the signals thus supplied to the thermal control grid of the thermally tunable oscillator tube are effective to produce changes agency of a thermal strut 4912.

Referring still to Fig. 4, the operation of the system there illustrated is as follows. During the time immediately after radar transmitter 4| is activated by closing switch 39 and until it has reached a substantial output level, no substantial signal will be supplied through waveguide section 42, attenuator 53, and waveguide section 54 to the auxiliary mixer crystal 55. Hence, no intermediate frequency output 'will be available for supply through I.-F. amplifier 5! to discriminator 62. Accordingly the bias applied through connection BI from load resistor 82 in the plate circuit of I.-F. amplifier tube 58 to the control grid of switch control tube 83 will be relatively positive. Therefore, by reason of the connections 86 and 81 from the plate and cathode of tube 83 respectively to the suppresser grids of switch tubes I9 and I9, if the values. of resistors 86;: and 87a are appropriatelyselected, signals applied to the control grid of switch tube I9 will appear across resistor I2I but signals applied to the control grid of switch tube I8 will not. Hence,

signals from the output of sweep multivibrator I II will be supplied through connection I20 to the control grid of switch tube I9 and thence from the plate of tube I9 through transmission line I22 'to the thermal control grid of local oscillator tube 49. These signals will, depending upon the condition of sweep multivibrator III, cause the frequency of the output from local oscillator tube 49 either to increase or decrease. In one of these conditions, multivibrator tube IIB will conduct; while, in the other condition, it will be cut off. The plate potential of tube I I6, when it t is conductive, will be more negative than when it is cut oif. Hence, when tube I I6 is conductive, the potential applied to the thermal control grid of local oscillator tube 49 will be positive and amaze resonator 98 is tuned. When this occurs, resonator 98 will be shock-excited in response to the output, and crystal I09 will respond to the oscillations in the resonator to develop a negative potential across resistor I I I. This potential,acting through tubes H2 and H8 on tubes H5 and H6 of sweep multivibrator I I1, will cause tube I I5-to conduct and tube II6 to cut off. The output fromsweep multivibrator 1, derived from .the plate of tube IIB, will then be relatively positive and, when applied through connection I20, tube 19'and'transmission line I22, will cause the thermal control grid of local oscillator tube 49 to become more negative, thereby commencing-a decrease in the frequency of the output of local oscillator tube 49. When the frequency of such output has reached the resonant frequency of low. frequency cavity resonator 91, coupled to the output of 'local oscillator tube 49, resonators! will be shock-excited in response thereto and crystal diode-99 will respond to the oscillations in resonator 9'! and will produce a positive potential across'resistor I I I. This potentiaLoperating through control tubes H2 and H8 upon multivibrator tubes H5 and H6, will cause tube II5 to out off and tube II6 to conduct. Then the output derived from the plate of multivibrator tube 'IIB will be relatively negative and a positive potential will be applied through transmission line I22 to the thermal control grid'of local oscillator'tube 49 so as to initiate an increase in the frequency of the output therefrom.

As soon as radar transmitter lI reaches a substantial level of output, such output will be supplied through waveguide section 42, attenuator 53 and waveguide section 54 to the auxiliary mixed crystal 55. Through the cooperation of crystal 55 this signal will be mixed with the output from local oscillator 49 supplied through waveguide section 56 to produce an intermediate frequency signal more-or-less approximating the desired intermediate frequency. This signal, supplied through I.-F. amplifier 51 to the control grid of I.-F. amplifier tube 58 will cause an increase in the plate current thereof and a corresponding increase in the negative bias applied through connection 8| from load resistor 82 to the control grid of switch control tube 93. The increase in the plate current of tube 58 and in the bias applied to tube 83 will be greater the more nearly the intermediate frequency approaches the desired value. As hereinbefore mentioned, I.-F. signal is app-lied to the grid of discriminator input tube 58 only during intervals corresponding to the transmission of radar pulses. Accordingly, in order that the bias applied to the grid of switch control tube 83 may be maintained during the intervals between the transmission of such pulses, there must be some provision for integrating the potential developed across load resistor 82. As already stated, such integration is provided for by appropriate selection of the time constants of R. C. circuits 82, 88 and 89, 99. The relatively large negative bias applied to the control grid of switch control tube 83, when radar transmitter M is operating and the output from local oscillator 49 is of nearly the proper frequency to produce the desired intermediate frequency signal, will cause the potential applied through connection 86 to the suppressor grid of switch tube 18' to be relatively positive, and that applied through connection 8! to the suppressor grid of switch tube I9 to be relatively negative Hence signals applied to the control grid of switch tube I8 will be caused to appear across load resistor I2 I, and those applied to the control grid of switch tube 19 will not. As a'result of this, local oscillator tube 49will no longer be controlled in response to the output from sweep multivibrator II'I, but instead will be controlled in response to an automatic frequency control voltage developed across condenser II and applied through connection I! to the control grid of tube 18/ If the potential across condenser II becomes more positive, the potential applied from the plate of switch tube I8 to the thermal control grid of the local oscillator tube 49 will become more negative and will effect a reduction in the frequency of the output from local oscillator 49. On the other hand, if the potential across condenser lI becomes more negative, the potential applied to the thermal control grid of local oscillator tube 49 will become more positive and the frequency of the output from the local oscillator tube will tend.

to increase.

With respect to the operation of discriminator 62 and gate and integrator circuit 69, if the intermediate frequency signal applied to the grid of amplifier tube 58 is higher than desired, a negative potential will be developed across discriminator load resistor 64, which will be applied through amplifier to connection 65 between the cathode of gate tube 81 and the plate of gate tube 68. This will cause tube 61 to conduct-and thereby decrease the potential across condenser II. For the reasons hereinbefore mentioned, this will efiect an increase in the frequencyof the output from local oscillator 49, which will tend to restore the intermediate frequency to'the desired value. Similarly, if the intermediate frequency signal is too low, a positive potential will appear across discriminator load resistor 63 which, when applied through amplifier 65 to connection 66, will cause tube 68 to conduct andincrease the potential across condenser I I. This, in turn, will reduce the potential applied to the 'grid of local oscillator tube 49 and will'c'ause the frequency of the output therefrom to decrease in a manner to restore the intermediate frequency to the desired value. As hereinbefore indicated, gate and integrator circuit 69 is employed to' maintain the afc voltage devel oped across condenser II during the intervals between transmitted radar pulses when no discriminator output is available.

It will be apparent that the frequencies to which'cavity resonators 91 and98 are tuned may be" appropriately selected so as to cause the frequency'oflocal oscillator 49, during the period of 'warm-upbf radar transmitter 41, to vary throughout a suitable range, as determined by the probable range of variation of the transmitter output frequency, so that, when radar transmitter 4i reaches its normal output level, some frequency within this range will correspond to the appropriate local oscillator frequency for mixing with'the radar transmitter frequency to yield the desired intermediate frequency. On the other'hand, the resonant frequencies of cavity resonators'9l and 98 may be selected so that this range'does 'not include frequencies which, when mixed with-the output from the radar transinitter, would yield an intermediate frequency such as to cause instability of operation of discrimin'aor 62 for'the reasons hereinbefore outw k o v M 'Finallv it will be noted that a system according to the invention, as exemplified in the embodiment of Fig. 4, will operate not only to cause the frequency of the local oscillator to sweep throughout a predetermined desired range during the period of Warm-up of the radar transmitter and until the usual automatic frequency control circuits become operative, but it will also be effective after the transmitter is operating normally and when, for any reason, the I.-F. signal departs in frequency by a substantial amount from the desired value, so that the usual automatic frequency control circuits cease to function. When this occurs the bias applied through connection 8| to switch control tube 83 will become less negative, and switch 80 will be controlled thereby so as to permit sweep multivibrator l I! to control the frequency of local oscillator 49 until the intermediate frequency has been restored to the desired range, whereupon the discriminator 62 will again take over control of the local oscillator in the manner hereinbefore described.

I claim:

1. In an automatic frequency control system for a superheterodyne receiver, a local oscillator whose frequency is controllable in response to a control voltage, a converter supplied with signals from said local oscillator and responsive to modulated carrier wave signals to produce correspondingly modulated intermediate frequency signals, a frequency discriminator supplied with said intermediate frequency signals and operative to produce a first control signal in response to deviations of said intermediate frequency from a predetermined reference value when said intermediate frequency lies within a predetermined range, a pair of cavity resonators separately supplied with signals from said local oscillator, the frequency bandwidths of said resonators being sufficiently small with respect to the separation between their respective resonant frequencies so that the response of each of said resonators is substantial only at frequencies for which the response of the other of said resonators is negligible, a generator of an alternating control signal for cyclically varying the frequency of said local oscillator thereby to cause said intermediate frequency to sweep through said operating frequency range of said discriminator, means for deriving detected signals from said cavity resonators and for applying said detected signals to said control signal generator to effect reversals in the sense of variation of said alternating control signal, switch means normally supplying said alternating control signal to said local oscillator to vary the frequency thereof, and means responsive to said control signal generated by said discriminator for actuating said switch to supply said last-named control signal to said local oscillator in place of said alternating control signal from said control signal generator.

2. In a radar system employing a superhetero- 'dyne receiver, a source of transmitted carrierwave signals whose frequency is susceptible to variation between two predetermined limits, a

local oscillator for generating a signal whose freresonant frequency of one of said resonators ly-.

ing without said limits of frequency variation of said carrier wave signals and differing from the nearest of said limits by an amount at least as great as said reference value of intermediate frequency, the resonant frequency of the other of said resonators differing from said last-named frequency limit by an amount less than said reference value of intermediate frequency, the frequency bandwidths of said resonators being sufficiently small with respect to the separation between their respective resonant frequencies so that the response of each of said resonators is substantial only at frequencies for which the response of the other of said resonators is negligible, a generator of an alternating control signal for cyclically varying the frequency of said local oscillator thereby to cause said intermediate frequency to sweep through said operating frequency range of said discriminator, means for deriving detected signals from said cavity resonators and for applying said detected signals to said control signal generator to eifect reversals in the sense of variation of said alternating control signal, electrically actuatable switch means normally supplying said alternating control signal to said local oscillator to vary the frequency thereof, and means responsive to said control signal generated by said discriminator for actuating said switch means to supply said last-named control signal to said local oscillator in place of said alternating control signal from said control signal generator.

ALLEN C. NIUNSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

